This invention relates to aluminium reduction cells, and particularly to the problem of cathode current collection therein. These cells are of the kind in which the electrolyte is molten cryolite Na.sub.3 AlF.sub.6 containing dissolved alumina Al.sub.2 O.sub.3, and electrolysis is performed between an anode suspended in the electrolyte and a cathode at the floor of the cell. In conventional cells, the floor is of carbon in which are embedded steel members connected to the external electricity supply. The carbon potlining transmits the electric current to the steel connecting members; but carbon is a rather poor electrical conductor, with the result that the cell voltage is higher than would be the case if a better cathode current collector were used.
U.S. Pat. No. 3,093,570 (Dewey) and British 2065174 (Odek) both show cathodes of titanium diboride TiB.sub.2 mounted in aluminium slabs for connection to the external electricity supply. TiB.sub.2 is a better electrical conductor than carbon; but it is expensive and difficult to form, and has low mechanical strength and a coefficient of thermal expansion very much greater than that of carbon or alumina or other potlining material. For these reasons, solid TiB.sub.2 cathodes have not achieved any significant commercial success.
It would be convenient and cheap to use cathode current collectors of aluminium metal. The fact that aluminium melts (660.degree. C.) far below the normal cell operating temperatures (950.degree.-980.degree. C.) means that the high-temperature end of such collectors would be fluid, but that does not in principle make them unsuitable. In practice however, it is found that thermal convection and magnetic effects cause efficient stirring of the molten metal and downward movement of the liquid-solid boundary, to the extent that such collectors cannot be used unless special precautions are taken.
In U.S. Pat. No. 3,607,685 (Johnson) there are described various designs of cathode current collector which are intended to overcome these difficulties. One design comprises an outer refractory tube containing a number of parallel refractory rods or fibres surrounded by molten aluminium; the rods or fibres, which are intended to restrain molten metal circulation, may be made of or coated with a material which is wet by aluminium metal. Another design uses aluminium alloys that have higher melting points and higher viscosities than commercial primary aluminium. Yet another design uses conductor assemblies each comprising a refractory tube and an aluminium core conductor, the high-temperature end of each being positioned at the bottom of a bowl-shaped depression in the cell potlining.
The cell electrolyte is replenished at intervals with alumina. For that purpose the frozen crust is broken at intervals and in the course of such crustbreaking, relatively large lumps of frozen crust, containing a high proportion of alumina, frequently fall into the bath. Because such lumps are of a density close to or even exceeding the density of the product metal they may penetrate the molten metal cathode layer. As the lumps of crust melt they form a sludge layer in the bottom of the cell beneath the molten metal. The sludge is believed to form discontinuous deposits on the cell floor, since the presence of sludge in a conventional cell leads to only a small increase in the cell voltage, although the electrical resistance of the sludge is quite high in relation to the electrical resistance of molten aluminium. It is therefore believed that the passage of the cathode current to the cathodic floor is through molten metal in direct contact with such floor.
In the practical operation of a standard electrolytic reduction cell for the production of aluminium it is found that the sludge content of the cell remains substantially constant and it is believed that the solid alumina in the sludge slowly dissolves in the electrolyte and migrates back to the electrolyte via the surface of the frozen electrolyte, which is present at the cell walls in conventional reduction cells, since the liquid components of the sludge can wet the surface of the frozen electrolyte. As already indicated the presence of sludge in conventional electrolytic reduction cells does not lead to severe operational problems.
However, in some circumstances sludge can cause operational problems. European patent specifications 68782 and 69502 are concerned with two such problems. No. 68782 provides a product metal tapping filter of a material that is wettable by the molten metal in preference to the electrolyte having apertures sized to permit flow of molten metal but to retain molten electrolyte and sludge. No. 69502 provides on the cell floor a monolayer of shapes of a material that is wettable by the molten metal in preference to the electrolyte, apertures in or between the shapes being sized to prevent entry of electrolyte or sludge.
The present invention is concerned with another such problem. When depressions are provided in the potlining, it has been realised that sludge is likely to collect in them. If cathode current collectors are sited at the bottoms of the depressions, it is likely that the collected sludge will rapidly increase the cell resistance. It is an object of the invention to overcome this problem.